The present invention relates generally to a method and apparatus for measuring aqueous erosion and transport from a sediment core sample and, in particular, to an adjustable shear stress erosion and transport flume having a variable depth sediment core sample.
Many contaminants are sorbed to sedimentary particles and are buried at depths of up to several meters in the bottom sediments of rivers, lakes, estuaries, and near-shore areas of the oceans. An important question is whether these buried sediments (and associated contaminants) can be exposed and transported during large floods and storms. In order to answer this question, knowledge of the erosion and transport properties of sediments at high shear stresses (up to and exceeding 10 N/m2) and with depth through the sediment layer (up to and exceeding one meter) is needed.
To characterize the movement of sediments in aquatic systems one must not only have an understanding of the bulk erosion rates of sediments, but also be able to distinguish between two primary modes of sediment transport, i.e., suspended transport and bedload transport. As shown in FIG. 1, suspended transport of a sediment grain or particle in flowing water occurs when the vertical component of the turbulent flow velocity is approximately equal to or greater than the settling (i.e., falling) speed of the grain. Unsuspended transport, also known as bedload transport, includes a variety of transport mechanisms, such as saltation, rolling, sliding, and tumbling. Saltation occurs when a particle momentarily leaves the bed and rises no higher than a few grain diameters. Rolling, sliding, and tumbling are additional processes wherein particles are transported along the bed primarily by the horizontal force of the overlying flow of water. In bedload transport, the particles receive no significant upward impulses other than those due to successive contacts between the solid and the bed; the fluid impulses on the grains being essentially horizontal. Saltation transport is generally included in bedload transport since saltation is restricted to only a few grain diameters in height above the bed.
Erosion of sediments in river and streambeds, along ocean beaches, harbors, and waterways, and around bridge support structures, is a complex process that depends on many variables. Sediments may erode particle-by-particle (e.g., sand and gravel), or may erode as aggregates or chunks, especially if the particles are fine-grained and cohesive (e.g., clay or silt). The chunks can vary in size from microns to centimeters, generally do not re-suspend, and are made from very fine-grained particles that would re-suspend if disaggregated. Sediments may also be contaminated with chemical, biological, or industrial contaminants, which can affect the degree of cohesiveness. Erosion and transport rates can also depend on grain size, shape, density, degree of cohesiveness, chemistry, organic content, and gas content. As shown in FIG. 2, aggregated (cohesive) particles eroded from the bed at an upstream position, X1, can de-aggregate at a downstream position, X2, due to subsequent impacts and collisions with the channel bottom and/or other aggregates and particles (e.g., during saltation transport). Erosion rates and transport modes also depend on the shear stress applied across the sediment""s surface by velocity of the flowing liquid (e.g., water). Typically, a threshold exists where no appreciable erosion occurs below a critical shear stress.
Accurate prediction of erosion rates (bulk or total, suspended, and bedload) and subsequent transport and re-deposition for each mode of transport (suspended or bedload) is complicated by a lack of understanding of the cohesive forces that bind together fine-grained sediments (especially for contaminated sediments). Therefore, a need exists for an apparatus and method that can accurately measure the individual contributions to the total erosion rate of sediments from suspended and bedload erosion processes, whether in the laboratory or in the field.
There is an existing apparatus for measuring bulk erosion of sediments (without transport), called a xe2x80x9cSEDflumexe2x80x9d, which is described in xe2x80x9cMeasurements of Erosion of Undisturbed Bottom Sediments with Depthxe2x80x9d, J. McNeil, C. Taylor, and W. Lick, Journal of Hydraulic Engineering, June, 1996. A similar device is also described in U.S. Pat. No. 6,260,409 to Briaud, et al., xe2x80x9cApparatus and Method for Prediction of Scour Related information in Soilsxe2x80x9d. However, these devices can only measure the total (i.e., bulk) erosion rate of a sediment core sample without any downstream transport, and, hence, cannot independently measure the separate contributions from suspended and bedload erosion while being transported down a channel.
Against this background, the present invention was developed.
The present invention relates to a method and apparatus for measuring the total erosion rate and downstream transport of suspended and bedload sediments using an adjustable shear stress erosion and transport (ASSET) flume with a variable-depth sediment core sample. Water is forced past a variable-depth sediment core sample in a closed channel, eroding sediments, and introducing suspended and bedload sediments into the flow stream. The core sample is continuously pushed into the flow stream, while keeping the surface level with the bottom of the channel. Eroded bedload sediments are transported downstream and then gravitationally separated from the flow stream into one or more quiescent traps. The captured bedload sediments (particles and aggregates) are weighed and compared to the total mass of sediment eroded, and also to the concentration of sediments suspended in the flow stream.